1. Field of the invention
The present invention relates to a composition of dielectric ceramics for use in a microwave band range.
2. Description of the Prior Art
In recent years much attention has been paid to a miniaturization of a movable communication apparatus using radio waves of a microwave band such as car telephones and portable telephone in association with the developments in the satellite broadcasts. In order to satisfy these requirements, it is necessary to miniaturize each of the components constituting the communication apparatus.
A dielectric body is assembled as a resonator in an oscillator or a filter device of the communication apparatus. The size of the resonator is in reverse proportion to the root of the dielectric constant of the dielectric body as long as the same resonating mode is used. Therefore, the miniaturization of the dielectric resonator requires a dielectric body having a high dielectric constant. In addition, in order to put the dielectric resonator into practical use, it is necessary to obtain a low loss, that is, a high Q value at a microwave band and further, a low temperature coefficient of the resonant frequency.
There have been recently developed various dielectric bodies for use in a resonator. For example, the U.S. patent Publication No. 4,330,631 disclosed a dielectric body of a BaO-TiO.sub.2 -Sm.sub.2 O.sub.3 system having a high dielectric constant. The dielectric body of this system has a relative dielectric constant of about 80, a Q value of about 3000 at frequency of 2 to 4 GHz and a low temperature coefficient of resonant frequency.
A further miniaturization of the resonator requires a dielectric body having a higher relative dielectric constant at a micro wave band, a higher no-load Q and a lower temperature coefficient of resonant frequency. On the other hand, there has been proposed another dielectric bodies having a higher dielectric constant. Among them, Japanese Patent Tokkaihei 1993-20925 discloses a dielectric body represented by the formula: EQU (Pb.sub.1-x Ca.sub.x).sub.1+a (Fe.sub.1/2 Nb.sub.1/2)O.sub.3+a
having a high dielectric constant of 90 or more and practical Q value and temperature characteristics. The sintering temperature is as low as about 1150.degree. C.
However, in the case of the dielectric body represented by the formula: EQU (Pb.sub.1-x Ca.sub.x).sub.1+a (Fe.sub.1/2 Nb.sub.1/2)O.sub.3+a
determination of x causes determination of the dielectric constant and the temperature coefficient of resonant frequency and the dielectric constant at .tau..sub.f =0 can not be changeable, so that such a dielectric body is not practical for use. FIG. 1 shows a variation of the dielectric constant corresponding to the temperature coefficient of resonant frequency at x variation of the formula: (Pb.sub.1-x Ca.sub.x).sub.1+a (Fe.sub.1/2 Nb.sub.1/2) O.sub.3+a system. In the case of the above system, the dielectric constant is fixed to 90 at .tau..sub.f =0, which value can not be adjusted to a lower one or a higher one. Accordingly, in order to make the system practical there is required a dielectric composition which dielectric constant and the temperature coefficient .tau..sub.f can be independently controlled. That is, there is a dielectric composition in which the variation curve of FIG. 1 can be shifted to a higher dielectric constant side or a lower dielectric constant side. In addition, in order to achieve a manufacturing process at a low cost, it is necessary to obtain a dielectric body which can be sintered at a lower temperature.